The virtual concatenation file of the files given on command line (or from
$stdin if no files were given).

$>

The default output for print, printf. $stdout by default.

$_

The last input line of string by gets or readline.

$0

Contains the name of the script being executed. May be assignable.

$*

Command line arguments given for the script sans args.

$$

The process number of the Ruby running this script.

$?

The status of the last executed child process. This value is thread-local.

$:

Load path for scripts and binary modules by load or require.

$“

The array contains the module names loaded by require.

$DEBUG

The debug flag, which is set by the -d switch. Enabling debug output
prints each exception raised to $stderr (but not its backtrace). Setting
this to a true value enables debug output as if -d were given on the
command line. Setting this to a false value disables debug output.

$LOADED_FEATURES

The alias to the $“.

$FILENAME

Current input file from $<. Same as $<.filename.

$LOAD_PATH

The alias to the $:.

$stderr

The current standard error output.

$stdin

The current standard input.

$stdout

The current standard output.

$VERBOSE

The verbose flag, which is set by the -w or -v switch. Setting this to a
true value enables warnings as if -w or -v were given on the command line.
Setting this to nil disables warnings, including from Kernel#warn.

$-0

The alias to $/.

$-a

True if option -a is set. Read-only variable.

$-d

The alias of $DEBUG. See $DEBUG above for further discussion.

$-F

The alias to $;.

$-i

In in-place-edit mode, this variable holds the extension, otherwise nil.